Ceramic membranes have a variety of industrial and scientific uses, the most common of which is use in separation processes. Organic membranes are often currently used in industry for separation processes, but ceramic membranes offer several advantages over organic membranes. Ceramic membranes are more resistant than organic membranes to organic solvents, chlorine, and extremes of pH. Ceramic membranes are also inherently more stable at high temperatures, thus allowing more efficient sterilization of process equipment than is possible with organic membranes. Ceramic membranes are generally quite resistant to microbial or biological degradation, which can occasionally be a problem with organic membranes. Ceramic membranes are also more mechanically stable under high pressures.
The mechanism of operation and types of separations which can be achieved by ceramic membranes are discussed in general by Asaeda et al., Jour. of Chem. Eng. of Japan, 19[1]:72-77 (1986). At least one line of ceramic filters is currently marketed under the trade name "Ceraflo" by the Norton Company of Worcester, Massachusetts.
Ceramic membranes may be formed in particulate or polymeric manners. Anderson, et al., J. Memb. Sci 39:243-258 (1988), describes different methods of making both particulate and polymeric sols from transition metal oxides. In general, particulate membranes have a smaller average pore diameter and a narrower pore size distribution as compared to polymeric membranes.
Particulate ceramic membranes are typically formed through a process beginning with metal-organic compounds. The compounds are hydrolyzed to form small metal oxide clusters which in turn aggregate to form metal oxide particles. The particles are fused into a unitary ceramic material. The gaps between the fused particles form a series of pores in the membrane.
The creation of these metal oxide ceramic membranes is generally conducted through a sol-gel procedure. Usually, the metal oxide is initiated into the process as a metal alkoxide solution. The metal is hydrolyzed to metal hydroxide monomers, clusters or particles, depending on the quantity of solvent used. The insoluble metal oxide particles are then peptized by the addition of an acid which causes the particles of the metal oxide to have a greater tendency to remain in suspension, presumably due to charges acquired by the particles during the peptizing process.
Such a sol can be evaporated to form a xerogel or simple gel, which is a semi-solid material. Further evaporation, and then sintering, of the gel results in a durable rigid material which can either be formed as an unsupported membrane or as a supported membrane coated onto a substrate. The substrate for the membrane can be either porous or non-porous and either metallic or non-metallic, depending on the particular application.
Two current limitations on the use of ceramic membranes are the elements which can be used to fabricate the membranes and the membrane pore size and range. With regard to the composition of the membranes, ceramic membranes have been created using many materials. For example, Leenaars et al., Jour. of Membrane Science, 24:261-270 (1985), report the use of the sol-gel procedure to prepare supported and unsupported alumina membranes. However, the sol-gel process used had inherent lower limits as to the size of the particles that could be produced in the sol thus making a lower limit on the size of the pores in the membranes. Ceramic membranes composed of titania, zirconia and other metal oxides have also been reported.
Investigators have previously investigated alumina membranes. In particular, Yoldas conducted significant research on the fabrication of gamma-alumina membranes made by a sol-gel process. Yoldas was able to achieve a relatively small particle size in the particles in the membranes and was able to make porous bodies. Yoldas, Jour. Mat. Sci., 12:6, pp. 1203-1208 (1977). In general, however, the method of Yoldas did not permit sufficiently fine control of the sol-gel process to result in useful uniformity of the particles, and the resulting membranes, so as to achieve useful filtration materials. For example, in his work with transparent alumina membranes, Yoldas reported that he was able to achieve materials that had small pores, having a radius less than 50 Angstrom, but only in materials which also had "larger river-like pores," having a significantly higher radius and contributing significantly to the porosity of the material. Yoldas, Cer. Bull., 54:3, 286-288 (1975).
In order for the materials to be useful for filtration, the pore size of the material needs to be within a relatively narrow range so that larger species are excluded from the fluid passing through the filter. It is also useful to make materials which have pore sizes of less than 20 Angstroms, which are particularly useful for certain applications. For example, for reverse osmosis, molecular sieving and gas separation applications, the ability to make porous membranes having a narrowly defined range of pore sizes, with the average size being less than 20 Angstroms, has significant advantage.